1. Field of the Invention
The invention relates to a process for removal of H.sub.2 S and CO.sub.2 from a gas mixture and preparation of a gas with a H.sub.2 S concentration suitable to be used in a Claus process.
2. Description of the Prior Art
In many cases it is necessary to remove H.sub.2 S and, if present, other sulfur-containing impurities such as COS from gas mixtures, for example to render these gas mixtures suitable for catalytic conversions using sulfur-sensitive catalysts, or in order to reduce environmental pollution if before or after combustion these gas mixtures are discharged to the atmosphere.
Examples of gas mixtures from which H.sub.2 S and/or other sulfur-containing compounds generally have to be removed are gases obtained by partial combustion, or complete or partial gasification of oil and coal, refinery gases, town gas, natural gas, coke-oven gas, water gas, propane and propylene.
In many cases the H.sub.2 S will be removed from the gas mixtures using liquid solvents, which will often have a basic character. In a large number of cases the gas mixtures from which H.sub.2 S is to be removed will also contain CO.sub.2, at least some of which will be absorbed in the liquid solvent together with H.sub.2 S. The H.sub.2 S and CO.sub.2 will be removed from the said gas mixtures at the pressure of the gas mixture concerned, i.e., in many cases at elevated pressure.
Although in most cases H.sub.2 S is to be removed nearly completely from the gas mixtures for the reasons given above, in many instances part or all of the CO.sub.2 may remain in the gas mixtures after the removal of the H.sub.2 S therefrom, because CO.sub.2 up till a certain extent does not hamper further use of the purified gas mixture. For that reason it would be attractive to have a process in which the ratio of H.sub.2 S to CO.sub.2 removed from the gas mixture can be controlled at will, or in other words, in which the selectivity of the removal of H.sub.2 S over CO.sub.2 can be regulated at will.
In many H.sub.2 S removal processes known hitherto the removal of COS often gives rise to problems, because the COS is not absorbed to a great extent in the solvent. A method is also needed to overcome this problem.
The H.sub.2 S- and CO.sub.2 -containing gas obtained after regeneration of the solvent used for the removal of these so-called acid gases from the gas mixture cannot be discharged to the atmosphere before at least most of the H.sub.2 S has been removed therefrom. The H.sub.2 S is very suitably removed from this gas by converting it to elemental sulfur, which is separated off. The conversion of H.sub.2 S into elemental sulfur is generally carried out by means of a Claus process in which some of the H.sub.2 S is oxidized to SO.sub.2, and sulfur and water are formed by reaction of H.sub.2 S with SO.sub.2, with or without the assistance of a suitable catalyst. In order to be able to carry out a Claus process, the molar percentage of H.sub.2 S in a mixture with CO.sub.2 must be at least about 15. If this percentage is between about 15 and about 40, the Claus process can be carried out by separating one-third part of the gas and combusting the H.sub.2 S therein to SO.sub.2, and subsequently mixing the resultant SO.sub.2 -containing gas with the rest of the H.sub.2 S-containing gas, after which the Claus reaction can be further carried out at elevated temperature and preferably in the presence of a catalyst. In case a Claus process is to be carried out with a gas which contains CO.sub.2 and about 40% mol. H.sub.2 S or more, the gas can be combusted with a quantity of air which is sufficient to convert one-third of the H.sub.2 S into SO.sub.2, and subsequent reaction of the H.sub.2 S and SO.sub.2 to form sulfur and water preferably in the presence of a suitable catalyst.
Besides the suitability of a process for the removal of H.sub.2 S and CO.sub.2 (and if desired COS) from gas mixtures, a feature of very much technical interest is the amount of energy needed for the process. The major amount of energy needed is used in the regeneration of the loaded solvent, which regeneration is in most cases carried out by stripping with steam. The steam requirement is greatly dependent on the amount of solvent circulating in the process, the lower the solvent circulation the lower the amount of steam needed for regeneration. The extent of loading of the loaded solvent to be regenerated may also be of importance in this respect. A reduction in solvent circulation would also contribute further savings from a capital cost point of view in that smaller absorption and regeneration columns are required, smaller heat-exchange equipment is needed and less solvent for filling the columns is necessary.